1. Field of the Invention
The present invention finds its application in the field of impressed current cathodic protection of reinforced concrete structures.
2. Prior Art and Other Conciderations
Cathodic protection of reinforced concrete is used to prevent or to stop corrosion of metallic reinforcements.
The most typical application field is in bridges, slabs, beams, piers, multi-story parkings, garages, etc,--situated in cold regions, where corrosion is caused by de-icing salts, as well as in structures exposed to marine environment.
This kind of systems is realized either using an anode structure typically having a net arrangement (i.e. titanium activated by noble metal oxides) or applying conductive coatings.
The main characteristic of any cathodic protection system in reinforced concrete structures is to guarantee that the protection conditions be extended to the whole surface of reinforcements, without reaching over-protection conditions.
The former condition obviously applies to any kind of structures; the latter, though being important in the protection of standard reinforced concrete structures, becomes mandatory where pre-stressed or post-tensioned concrete elements are present and have to be protected. In fact, steels used for this type of structures exhibit very high mechanical characteristics, generally not lower than 1400 MPa, making them extremely exposed to the risk of hydrogen embrittlement phenomena.
This means that, in a cathodic protection system applied to a pre-stressed or post-tensioned reinforced concrete structure, if the potential of these steels gets below the threshold value, in correspondence of which the hydrogen evolution reaction becomes appreciable, hydrogen embrittlement may occur.
For the steels generally used in these structures, this threshold value ranges around -0.9 V with respect to Ag/AgCl electrode (W. H. Hartt, P. K. Narayanan, T. Y. Chen, C. C. Kumira, "Cathodic Protection and Environmental Cracking of Prestressing Steel", CORROSION/89, paper n. 382, NACE, New Orleans, April 1989; R. N. Parkins et al., "Environmental Sensitive Cracking of Prestressing Steels", Corrosion Science 22, p. 379, 1982).
When using carbon or low alloy steels showing ordinary mechanical characteristics, no embrittlement is observed. A threshold value is however set around--1.1 V (The Concrete Society Tech. Report N, 36 "Cathodic Protection of Reinforced Concrete", London 1988). In fact, beyond this potential value, it is not advisable to operate, not only due to economic reasons, but also to avoid possible occurrence of reduced bond between concrete and reinforcement.
Traditional monitoring technique for cathodic protection is based on the following check operations:
1. protection conditions are reached (i.e. by means of the so-called 100 mV decay method, that consists in checking that the cathodic potential variation, in the first four hours after opening the circuit, exceeds 100 mV) and,
2. potential of reinforcements is always nobler than the above mentioned critical value, (i.e. 100 mV) so as to exclude any over-protection risks.
That criteria are not suitable for pre-stressed concrete and, in any case, also for ordinary concrete, cannot avoid overprotection condition in some points (for instance, where rebars are very close to the anode, in correspondence of zone of concrete where the resistivity becomes unexpectedly very low due to chloride contamination). In fact, the detection of possible over-protection conditions is strongly dependent on the location of reference electrodes (portable or fixed); it follows that only monitored zones (i.e. presence of reference eletrode) are controlled, that means a few percent of the protected area, and are not representative of the map of the potentials on the whole surface of reinforcements. In effect, the potential of the surface of reinforcements is not uniform, but varies from one to another point, depending on the position with respect to the anode surface and to surrounding reinforcements. The amount of the variations changes with operating and environmental conditions. For instance, during winter or in high atmospheric humidity conditions, for which the oxygen diffusion within concrete may be difficult, such local variations may be very high.